Carbon film and process for preparing the same

ABSTRACT

(1) A carbon film derived from a polyimide film, which has a tensile strength of at least 15 Kgf/mm 2 , a tensile modulus of elasticity of at least 5000 Kgf/mm 2  and an electric conductivity of at least 200 S/cm. 
     (2) A process for preparing a carbon film which comprises the following steps: 
     1) reacting a monomer combination selected from the group consisting of a combination comprising a tetracarboxylic acid dianhydride and an aromatic diamine and a combination comprising a tetracarboxylic acid dianhydride, an aromatic diamine, and a polyamino compound having at least three amino groups to form a polyamic acid; 
     2) forming a film of polyamic acid; 
     3) imidizing the polyamic acid film to form a polyimide film having a tensile strength of at least 10 Kgf/mm 2  and a tensile modulus of elasticity of at least 500 Kgf/mm 2  ; and 
     4) carbonizing the polyimide film in an inert gas or in a vacuum until a carbon film having a tensile strength of at least 15 Kgf/mm 2 , a tensile modulus of elasticity of at least 5000 Kgf/mm 2  and an electric conductivity of at least 200 S/cm is formed.

FIELD OF THE INVENTION

This invention relates to a carbon film having excellent mechanicalcharacteristics and high electrical conductivity, obtained byheat-treating a polyimide film at an elevated temperature. It alsorelates to a process for preparing the same.

BACKGROUND OF THE INVENTION

Previously, carbon materials have been widely used as electrodematerials, heating elements, structural materials, heat insulatingmaterials, heat-resistant sealing materials, X-ray parts, and the likebecause of their excellent heat resistance, chemical resistance, andelectrical conductivity. In particular, sheet-form carbon films areexpected to be applicable to important uses as industrial materials asdescribed above, and have been extensively studied in recent years.

Processes for preparing carbon films include a process in which a carbonfilm is prepared from natural graphite, a process in which a carbon filmis prepared by the high-temperature decomposition deposition of ahydrocarbon in a gas phase, and a process in which a carbon film isprepared by treating an organic material or a carbonaceous material atan elevated temperature. Carbon films obtained from these processes arepresently being applied to fields which utilize their excellentcharacteristics, such as heat resistance, chemical resistance, andelectrical conductivity.

Among these processes for preparing carbon films, processes forobtaining a carbon film by heat-decomposing a polymer film have beenextensively studied because of their simplicity, and many attempts havebeen made to improve the process as disclosed in JP-A-53-139676 (theterm "JP-A" as used herein means an "unexamined published Japanesepatent application"), JP-A-60-11215 (corresponding to U.S. Pat. No.4,599,193), JP-A-60-60-235709, JP-A-62-91414 (corresponding to U.S. Pat.No. 4,915,984) and JP-A-1-105199 (corresponding to U.S. Pat. No.4,842,665). Among the processes of the type described above, a processin which an aromatic polyimide film is used as the polymer film, whichis heat-decomposed by heat-treating it at an elevated temperature toobtain a carbon film, is described in, for example, JP-B-64-12305 (theterm "JP-B" as used herein means an "examined Japanese patentpublication"). A process in which a polymer film is stretched and thencarbonized is disclosed in, for example, JP-B-1-48204 (corresponding toU.S. Pat. Nos. 4,626,588 and 4,791,177).

Conventional carbon films exhibit satisfactory electrical conductivityor mechanical properties. However, a carbon film which exhibits bothimproved electrical conductivity and mechanical properties has not yetbeen produced. A carbon film having a tensile strength of at least 15Kgf/mm², a tensile modulus of elasticity of at least 5,000 Kgf/mm² andan electric conductivity of at least 200 S/cm (S=mho), has not beenpreviously produced.

For example, a graphite layer should be orderly arranged in the film byheat-treating the film at a temperature of at least 3000° C. in anoxygen-free atmosphere under high pressure to obtain a carbon film whichexhibits excellent electrical conductivity. However, it is difficult tomanufacture an apparatus which is to be operated at such a hightemperature. In addition, carbon films obtained by such ahigh-temperature treatment are generally brittle, and do not alwaysexhibit excellent mechanical properties.

Further, there is no known process for preparing a carbon film having atensile strength of at least 15 Kgf/mm², a tensile modulus of elasticityof at least 5,000 Kgf/mm² and an electric conductivity of at least 200S/cm using aromatic polyimides as a starting material.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a carbon film derivedfrom an aromatic polyimide, which has improved mechanical properties,particularly an improved tensile strength and tensile modulus ofelasticity, as well as an improved electric conductivity.

Another object of the present invention is to provide a process forpreparing a carbon film having an improved tensile strength and tensilemodulus of elasticity and an improved electric conductivity from anaromatic polyimide.

The carbon film of the present invention is derived from an aromaticpolyimide and has a tensile strength of at least 15 Kgf/mm², a tensilemodulus of elasticity of at least 5,000 Kgf/mm², and an electricconductivity of at least 200 S/cm.

The process for preparing the carbon film of the present inventioncomprises reacting a monomer combination comprising a tetracarboxylicacid anhydride (component (A)) and an aromatic diamine (component (B))to form a polyamic acid, forming a polyamic acid film, and imidizing thefilm by a dehydration-cyclization reaction to form a polyimide filmhaving a tensile strength of at least 10 Kgf/mm² and a tensile modulusof elasticity of at least 500 Kgf/mm2; and further heat-treating thefilm in an inert gas or in a vacuum to obtain a carbon film having atensile strength of at least 15 Kgf/mm², a tensile modulus of elasticityof at least 5,000 Kgf/mm² and an electric conductivity of at least 200S/cm.

A carbon film of the present invention may also be obtained from themonomer combination as described above and which further contains atleast one polyamine compound (component (C)) having at least three aminogroups.

DETAILED DESCRIPTION OF THE INVENTION

Typical examples of the tetracarboxylic acid dianhydride used ascomponent (A) in the present invention include a tetracarboxy benzenedianhydride, a tetracarboxy dianhydride of a compound having 2 to 5condensed benzene rings, and compounds represented by formula (I), andsubstituted compounds thereof: ##STR1## wherein R₁ represents --O--,--CO--, --SO₂ --, --SO--, an alkylene group, an alkylene bicarbonyloxygroup, an alkylene bioxycarbonyl group, a phenylene group, a phenylenealkylene group, or a phenylene dialkylene group; n₄ is 0 or 1; n₅ is 0or 1; and n₆ represents 1 or 2, provided that the sum of n₅ and n₆ is 2.

Examples of the tetracarboxylic acid dianhydrides include pyromelliticdianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride,3,3',4,4'-biphenyltetracarboxylic acid dianhydride,2,3,3',4'-biphenyltetracarboxylic acid dianhydride,2,2',3,3'-biphenyltetracarboxylic acid dianhydride,2,2',6,6'-biphenyltetracarboxylic acid dianhydride,2,3,6,7-naphthalenetetracarboxylic acid dianhydride,1,2,5,6-naphthalenetetracarboxylic acid dianhydride,2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,bis(3,4-dicarboxyphenyl)sulfone dianhydride,bis(3,4-dicarboxyphenyl)ether dianhydride,3,4,9,10-perylenetetracarboxylic acid dianhydride,naphthalene-1,2,4,5-tetracarboxylic acid dianhydride,naphthalene-1,4,5,8-tetracarboxylic acid dianhydride,benzene-1,2,3,4-tetracarboxylic acid dianhydride and ethylene glycolbis(anhydrotrimellitate). These compounds may be used either alone or asa mixture of two or more.

Typical examples of the aromatic diamine used as component (B) are aphenylene diamine, a diamino pyridine, a diamino compound having 2 to 8condensed benzene rings, compounds represented by formula (II), andsubstituted compounds thereof: ##STR2## wherein R₂ represents --O--,--CO--, --SO₂ --, --SO--, a phenylene group, an alkylene group, aphenylene alkylene group, a n₇ is 0 or 1; R₃ represents --O--, --CO--,--SO₂ --, --SO--, a phenylene group, an alkylene group, or a phenylenealkylene group, a phenylene dialkylene group; and n₈ is 0 or 1.

Examples of the aromatic diamine include m-phenylenediamine,p-phenylenediamine, benzidine, o-toluidine, benzidine derivatives suchas 3,3'-dichloro-4,4'diaminobiphenyl, 4,4'-diaminodiphenylmethane and4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylpropane,3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide,4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone,3,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl ether,3,4'-diaminodiphenyl ether; 4,4'-diaminobenzophenone,3,3'-diaminobenzophenone, 2,2'-bis-(4-aminophenyl)propane,3,3'-diaminobiphenyl, 2,6-diaminopyridine, 2,5-diaminopyridine,3,4-diaminopyridine, bis[4-(4-aminophenoxy)phenyl]sulfone,bis[4-(4-aminophenoxy)phenyl]ether, bis[4-(3-aminophenoxy)phenyl]ether,2,2'-bis[4-(4-aminophenoxy)phenyl]propane,2,2'-bis[4-(3-aminophenoxy)phenyl]propane,4,4'-bis(4-aminophenoxy)biphenyl, 1,4-bis(4-aminophenoxy)benzene,1,3-bis(4-aminophenoxy)benzene,2,2'-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane,1,5-diaminonaphthalene, 2,6-diaminonaphthalene, 4,4'-diaminotriphenyl,4,4'-diaminotetraphenyl, 1,6-diaminonaphthalene, 1,4-diaminoanthracene,1,8-diaminopyrene and substituted compounds thereof.

Examples of polyamino compounds as component (C) are represented byformula (III) or (IV): ##STR3## wherein A₁ represents ##STR4## A₂represents ##STR5## n₁ is 0 or an integer of 1 to 4; n₂ is 0 or aninteger of 1 to 3; X represents an acid; q is the base number of theacid; R represents --O--, --CH₂ --, --CO--or --SO₂ --; and n₃ is 0 or 1;and A₁ and A₂ may be substituted.

The acid represented by X in formulae (III) and (IV) may be either anorganic or inorganic acid. Examples of the acid includep-toluenesulfuric acid, picric acid, and hydrochloric acid.

The substituents for the above described compounds represented byformula (I), (II), (III) or (IV) should not adversely affect thecondensation reaction and should not deteriorate characteristics of theproducts obtained therefrom. Examples of such substituents include analiphatic hydrocarbon group and a halogen atom (e.g., methyl, ethyl,propyl, butyl, isopropyl, tert-butyl, C1 and Br).

These monomers may be used either alone or as a mixture of two or moreof them.

Typical examples of the polyamino compounds used as component (C)include 3,3',4,4'-tetraaminodiphenyl ether,3,3',4,4'-tetraaminodiphenylmethane, 3,3',4,4'-tetraaminobenzophenone,3,3',4,4'-tetraaminodiphenyl sulphone, 3,3',4,4'-tetraaminobiphenyl,1,2,4,5-tetraaminobenzene, 3,3',4-triaminobiphenyl,3,3',4-triaminodiphenylmethane, 3,3',4-triaminobenzophenone,3,3',4-triaminodiphenylsulfone, and 1,2,4-triaminobenzene, and theirmono-, di-, tri- or tetra-acid salts such as3,3',4,4'-tetraaminodiphenyl ether tetrahydrochloride,3,3',4,4'-tetraaminodiphenylmethane tetrahydrochloride,3,3',4,4'-tetraaminobenzophenone tetrahydrochloride,3,3',4,4'-tetraaminodiphenyl sulfone tetrahydrochloride,3,3',4,4'-tetraaminobiphenyl tetrahydrochloride,1,2,4,5-tetraaminobenzene tetrahydrochloride,3,3',4-triaminodiphenylmethane trihydrochloride,3,3',4-triaminobenzophenone trihydrochloride,3,3',4-triaminodiphenylsulfone trihydrochloride, 3,3',4-triaminobiphenyltrihydrochloride, and 1,2,4-triaminobenzene dihydrochloride. The abovecompounds usually exist in the hydrated form. These compounds may beused either alone or as a mixture of two or more.

A polyamic acid which is used in the present invention is obtained bypolyaddition reaction of components (A) and (B) or components (A), (B)and (C). Upon reaction each of anhydride groups is opened and acarboxylic acid group and an amido group forming a polymer chain areformed.

The polyamic acid film can be obtained by casting or coating a polymersolution of the polyamic acid on a support.

When component (C) is used for preparation of a polyamic acid bysolution polymerization, a polyamic acid having a three-dimensionalnetwork molecular structure can be obtained in a gel form. Therefore, itis convenient to form a polyamic acid film prior to starting thegelation and then to proceed to gelation and completion of thepolymerization reaction. This is disclosed in U.S. patent applicationSer. No. 07/586,103 (filed Sep. 21, 1990) corresponding to EuropeanPatent 418889A.

When the above-described tetraamino compounds and triamino compoundsused as component (C) are not in the form of an addition salt, the timetaken for gelation is considerably reduced. Thus, it is preferred to usecomponent (C) in the form of an addition salt. The amount of theaddition salts is preferably 50 to 100 mol %, more preferably 75 to 100mol % based on the total moles of the polyamino compounds.

The thickness of the polyamic acid film is preferably from 1 to 1,000μm, and more preferably from 0 to 100 μm, to ensure easy handling.

Examples of support material include glass, metals, ceramics, andpolymer resins, such as a polyester resin.

The polyamic acid film formed on the support may be subjected to adehydration-cyclization reaction after peeling, or without peeling thefilm from the support.

The polyamic acid of the present invention can be obtained by reactingthe components (A) and (B), or (A), (B) and (C) in an organic solvent,generally at a temperature of -30° to 80° C., preferably -5° to 50° C.,more preferably 0° to 30° C., in an inert atmosphere. The reaction timeis not longer than 10 hours, preferably not longer than 6 hours, morepreferably not longer than 2 hours. When the reaction time exceeds 10hours, a hydrolysis reaction often occurs. Usually, the reaction time isat least 5 minutes, and preferably at least one hour.

It is necessary that the organic solvent used for the reaction be inertto the reaction and capable of dissolving the components (A), (B) and(C) to be reacted. Typical examples of the organic solvent includeN,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylformamide,N,N-diethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone,N,N-dimethylmethoxyacetamide, hexamethylphosphoamide, pyridine, dimethylsulfone, tetramethylene sulfone, phenols such as cresol, phenol andxylenol, benzene, toluene, xylene, benzonitrile, dioxane andcyclohexane. These solvents may be used either alone or as a mixture oftwo or more of them.

A film of a polyamic acid obtained from components (A) and (B) can alsobe formed by an optional method, for example, by casting and drying asolution of a polyamic acid on a support. Examples of the solvent forthe coating solution are the same as those recited for thepolymerization reaction above. The concentration of the polyamic acid inthe casting solution is preferably from 5 to 20 wt %.

Further, a polyamic acid film can be obtained in the following manner.The film of the polyamic acid, which contains the organic solvent, isimmersed in or washed with a coagulating liquid comprising a poorsolvent for the polyamic acid, whereby the organic solvent left in thefilm is removed by the substitution with the poor solvent. In thisoperation, water is generally used as a coagulating liquid. Alcoholssuch as methanol and ethanol may be used with water in an amount of notmore than 50% by weight based on the total amount of the coagulatingliquid. Further, ketones such as acetone, amide solvents such asN,N-dimethylacetamide and chlorine-containing solvents such as1,2-dichloroethane may be used in an amount of not more than 20% byweight based on the total weight of the coagulating liquid. When theabove-described washing of the film is conducted, impurities such aschlorine contained in the film can be removed sufficiently.

It is preferred from the viewpoint of handling, especially whencomponent (C) is also used, that the amount of the polyamic acid in thereaction mixture after completion of the reaction is from 3 to 50% byweight, preferably from 5 to 20% by weight, based on the total weight ofthe polyamic acid and solvent. When the concentration of the polyamicacid is lower than 3% by weight, the resulting polyamic acid gel ispoorly free-standing, while when the concentration exceeds 50% byweight, the solids content is too high, the viscosity of the solution isincreased during the course of the polymerization and the polymerizationreaction does not proceed as expected.

The molar ratio of component (A) to component (B) to component (C)(hereinafter referred to as (A)/ (B)/(C)) to be reacted preferably is100:60-100:0-20 (such ratios are hereinafter designated as follows:100/(60-100)/(0-20).

Generally, it is preferred that when the polyamic acid is prepared fromthe tetracarboxylic acid dianhydride and the aromatic diamine, bothcomponents are reacted in an equimolar ratio as much as possible toincrease molecular weight. Therefore it is preferable that the molarratio of (A)/(B) is 100/(60-100), more preferably 100/(70-100), and mostpreferably 100/(80-100) When component (C) is also used the molar ratioof (A)/(B)/(C) is preferably 100/(60-100)/(1-20), more preferably100/(70-100)/(4-15), and most preferably (100)/(80-100)/(4-15) toincrease the degree of polymerization. Furthermore, the components (A),(B) and (C) preferably are used in proportions such that the differenceof the molar number of the reactive groups of the tetracarboxylic aciddianhydride (which is two per molecule of acid dianhydride) and thetotal molar number of the reaction groups of the aromatic diamine andthe polyamine compound is adjusted to a value within ±5%. That is, apreferred equivalent ratio of reactants is as follows: ##EQU1##Therefore, when a tetraamino compound is used as component (C) thefollowing relationship should be satisfied:

    0.95 ×[moles of (B)+2 ×moles of (C)]<moles of (A) <1.05 ×[moles of (B)+2 ×moles of (C)]

When the molar ratio of (C)/(A) is lower than 1/100, the formation ofthe three-dimensional network structure is insufficient, that is,gelation tends not to occur.

When the molar ratio of (C)/(A) is higher than 20/100, the reactionproceeds rapidly, the resulting gel is suspended as small pieces in thesolvent and a uniform product is difficult to obtain. Accordingly, whena polyimide film is prepared through gelation of the polyamic acid, themolar ratio of (C)/(A) preferably should be (1-20)/100.

The reaction of the components (A) and (B), or (A), (B) and (C) is apolymerization reaction of the carboxylic anhydride and amino compounds.The component (A) may be added to a solution of the component (B) orcomponents (B) and (C) dissolved in an organic solvent, in an inertatmosphere such as nitrogen gas. The component (A) may be added in theform of a solid or a solution thereof in a solvent. Alternatively, thecomponent (B) or the components (B) and (C) may be added to a solutionof the component (A) dissolved in an organic solvent. If desired, thecomponent (C) may be added during the course of the reaction of thecomponent (A) with the component (B). A still further alternative is toadd the component (B) to a reaction system where predetermined amountsof the components (A) and (C) are reacted, but when following thisalternative there is a possibility that the reaction will not proceedsufficiently and that gelation may not occur depending on preparationconditions.

The molecular weight of the polyamic acid is preferably 10,000 to300,000, more preferably 30,000 to 300,000.

The degree of crosslinking in the network molecular structure of thepolyamic acid can be increased by heating (usually 50° C. to 80° C.),subjecting to light (such as ultraviolet ray) or by applying pressure(usually up to 3.0 Kg/cm²).

In addition to the components (A) and (B) or components (A), (B) and(C), the reaction mixture may contain small amounts (usually up to 10 wt% based on the component (A)) of a di- or tricarboxylic acid, acarboxylic acid anhydride or a substituted compound thereof (e.g.,trimelitic acid, trimelitic acid anhydride, isophthalic acid dichloride,terephthalic acid dichloride and trimelitic acid dichloride).

The three-dimensional structure of the polyamic acid containing about 50to 97% (based on the total weight of the polyamic acid and the solvent)by weight of the above-described organic solvent is a free-standing gel.

The amount of the solvent in the three-dimensional structural product ofthe polyamic acid can be controlled by swelling or shrinkage by theabsorption or desorption of the solvent. Shrinkage (deswelling) can beconducted by heating preferably at up to 80° C. under reduced pressureor by solvent replacement using a poor solvent. The number of linkagesin the chemical structure of the gel can be increased by externalinfluences such as temperature, light and pressure. Swelling can beconducted by immersing the polyamic acid in an organic solvent, forexample at 0° to 80° C., for 1 to 60 minutes.

The polyimide film can be obtained by subjecting the polyamic acid filmto a conventional imidization which is carrying out by adehydration-cyclization reaction, i.e., by the following reaction:##STR6## For example, imidization can be conducted by a high-temperaturetreatment or a chemical dehydrating treatment using a dehydrating agent.

Methods for preparation of the polyimide which can be used in thepresent invention are disclosed in U.S. patent application Ser. No.07/586103 filed Sep. 21, 1990, EP 418889, U.S. Pat. No. 4,690,999,JP-A-63-178133, JP-B-46-20600, JP-B-44-19878, JP-B-44-26311, andEP-A-274603 (corresponding to JP-A-215726).

When imidization is to be conducted by a high-temperature treatment of apolyamic acid derived from components (A) and (B), the film of thepolyamic acid is dried until the amount of the solvent is reduced to 0to 10% by weight at a temperature of 50° to 200° C. for at least 10seconds prior to subjecting the film to a heat-treatment for conductingimidization.

When the polyamic acid is in a gel state, i.e., contains a solvent, thefilm is preferably dried at 30° to 80° C. for 0.5 to 10 hours, morepreferably at 30° to 50° C. for 1 to 5 hours until the solvent in thefilm becomes preferably 0 to 50% by weight, more preferably 40% byweight or less. Usually, the temperature is gradually elevated in orderto maintain the form of the film.

The polyamic acid film which is in a dried state (or in a deswollenstate) is subjected to heating for conducting the imidization. Theheating is usually conducted at 150° to 500° C. for 10 seconds to 10hours, preferably at 200° to 400° C. for 1 to 5 hours.

The heat treatment can be carried out while fixing both ends of thepolyamic acid film in the longitudinal direction of the film by means ofa fixing frame, a fastener or a pin guide, to obtain a film exhibitingexcellent dimensional stability and mechanical characteristics. Thismethod is very effective, because the film of the polyamic acid isshrunk by drying or heat treatment in particular.

The imidization can be confirmed by observing infrared spectralabsorption at 1780 cm⁻¹ and 1720 cm⁻¹.

The imidization of the polyamic acid can be conducted while retainingthe polyamic acid film in a wet state. For example, a method whereindehydration and cyclization are chemically carried out by impregnatingthe polyamic acid film with a solution of a dehydrating agent may beused. Alternatively, the polyamic acid film may be subjected to achemical treatment after the organic solvent is removed from the film orafter the organic solvent is replaced with a poor solvent.

It is effective to use acid anhydrides as dehydrating agents in thepresence of amines as catalysts in the chemical dehydration reaction.Examples of the acid anhydrides include aliphatic acid anhydrides suchas acetic anhydride, propionic anhydride and butyric anhydride andbenzoic anhydride. These compounds may be used either alone or as amixture of two or more.

Examples of the amines which can be used as catalysts for the chemicaldehydration reaction include tert-amines such as trimethylamine,triethylamine, triethylenediamine, hexamethylenetetraamine,tributylamine, dimethylaniline, pyridine, α-picoline, β-picoline,γ-picoline, isoquinoline and lutidine. At least one amine selected fromthe group consisting of tert-amines is used as a catalyst.

The amount of the acid anhydride to be added in the chemical dehydrationreaction is preferably 1 to 5 equivalents, more preferably 1 to 3equivalents per equivalent of carboxyl group present in the polyamicacid. The amount of the catalyst in the chemical dehydration reaction ispreferably 0.01 to 1.5 equivalents, more preferably 0.2 to 1 equivalentper equivalent of carboxyl group present in the polyamic acid.

The acid anhydride and the amine are dissolved in an organic solvent forthese compounds to form a solution used for the dehydration reaction.Examples of solvents include the above-described solvents for components(A), (B) and (C). The concentration of the acid anhydride is usually 1to 10% by weight based on the total weight of the polyamic acidsolution.

The chemical treatment for imidization is preferably conducted at 0° to80° C. for 0.5 to 48 hours.

The polyimide film which is used as the precursor of the carbon film inthe present invention has a high tensile strength, and a high tensilemodulus of elasticity.

In order to obtain carbon film of the present invention it is necessarythat the polyimide film have a tensile strength of at least 10 Kgf/mm²preferably at least 15 Kgf/mm², and more preferably at least 20 Kgf/mm²,and a tensile modulus of elasticity of at least 500 Kgf/mm² preferablyat least 800 Kgf/mm², and more preferably at least 1,000 Kgf/mm². If thepolyimide film has a lower tensile strength or tensile modulus ofelasticity than those described above, the film of the polyamic acid,which is the precursor of the polyimide, or the polyimide film must bestretched, preferably more than 1.0 times its original length. Thisallows the molecular chain of the polymer to be arranged mainly in oneor two (biaxial) direction, which increases the tensile strength and thetensile modulus of elasticity of the polyimide film. In the stage of thepolyamic acid film, the stretching of the film can be readily performed,and a highly oriented film can be obtained. The stretching can beconducted in one direction, or in two directions which are permitted notto be the same stretching ratio for the cross direction.

Stretching can be conducted by any conventional method, for example, bya tentering method and a roll method.

Stretching is usually conducted at a ratio of more than 1 times and upto 3 times, preferably from 1.2 to 2 times original length. In the caseof one direction stretching, stretching can be conducted to more than 3times original length, up to the ratio at which the film is destroyed.However, usually it is not necessary to conduct the stretching at aratio of greater than 5 times original length.

Any of conventional stretching methods including the tenter system androll system can be used. If an organic solvent capable of swelling thefilm or a liquid functioning as a plasticizer is contained in the film(polyamic acid film or partially imidized polyamic acid film), thestretching of the film is facilitated and a polyimide film having a highdrawing ratio can be prepared. The partially imidized polyamic acid filmcan be obtained by imidizing polyamic acid preferably up to 50%, morepreferably up to 30% of amido groups by heating the polymer at 100° to150° C. for 30 to 120 minutes, or by treating the polyamic acid using adehydration agent in an amount of 1 to 50 equivalents based on theequivalent number of amido groups in the polymer at 0 to 50° C. for 1 to24 hours. For example, by adding 1 to 50 equivalents of an acidanhydride and an amine catalyst to a polyamic acid solution and bystirring the thus obtained reaction mixture for 3 to 12 hours, a partialimidization can be completed. The completion of a partial imidizationcan be determined by an IR measurement. Further imidization is conductedafter forming film.

Examples of the liquid capable of swelling the film includeN,N-dimethylformamide and N-methyl-2pyrrolidone which are used in thepolymerization reaction of the polyamic acid. Further, there can be usedphenols, hydrocarbons, ketones, alcohols and ethers.

Examples of the liquid which functions as plasticizer include ethyleneglycol and an esterified product of ethylene glycol (e.g.,diethyleneglycol diethylether and ethyleneglycol monoethylether).

The polyimide film obtained by a dehydration cyclization reaction of astretched polyamic acid film has improved tensile strength and tensilemodulus of elasticity (which is measured in the same direction as thatof the stretching or in the direction providing the highest values oftensile strength and tensile modulus of elasticity), and is preferred asthe precursor of a high performance carbon film.

As the tetracarboxylic acid dianhydride used in the present invention,it is preferred to use pyromellitic dianhydride orbiphenyltetracarboxylic acid dianhydride either singly or as a mixtureof two or more of the tetracarboxylic acid dianhydrides. Pyromelliticdianhydride and biphenyltetracarboxylic acid are preferred to obtain apolyimide film having high-tensile strength and high-tensile modulus ofelasticity. When these tetracarboxylic acid dianhydrides are used withother tetracarboxylic acid dianhydrides, it is preferable to use them inan amount of at least 80 mol %, more preferably at least 85 mol %, andmost preferably at least 90 mol % based on the total number of mols ofthe dianhydrides.

As the aromatic diamine, it is preferred to use p-phenylenediamine,benzidine, o-toluidine, benzidine derivatives such as3,3'-dichloro-4,4'-diaminobiphenyl, 4,4'-diaminodiphenylmethane and4,4'-diaminodiphenyl ether, either singly or as a mixture of two or moreof the aromatic diamines. These diamines are preferred to obtain ahigh-tensile strength, high-tensile modulus of elasticity polyimide filmwhich is the precursor of the carbon film of the present invention. Whenthese aromatic amines are used with other aromatic diamines, it ispreferable to use them in an amount of 80 mol %, more preferably atleast 85 mol %, and most preferably at least 90 mol % based on the totalnumber of mols of the diamines.

Among the polyamino compounds described above, the use of 3,3',4,4'-tetraaminobiphenyl tetrahydrochloride and 1,2,4-triaminobenzenedihydrochloride singly or as a mixture with other polyamic compounds, ispreferred to obtain a polyimide film having a high-tensile strength, anda high-tensile modulus of elasticity, which is the precursor of thecarbon film of the present invention. It is preferable to use thesecompounds in an amount of at least 10 mol %, more preferably at least 15mol %, and most preferably at least 20 mol % based on the total numberof mols of the polyamino compounds.

The polyimide film obtained from these preferred components has ahigh-tensile strength and a high-tensile modulus of elasticity becausesuch a polyimide has molecular chains which are rigid and highlycrystalline.

The polyimides may be prepared using one or more tetracarboxylic aciddianhydrides and one or more aromatic diamines as the monomercomponents, or the polyimides may also be prepared by further using oneor more polyamino compounds, and hence the resulting polymer may be acopolymer. Further, the polyimide may be in the form of a blend ofpolyimides comprised of different monomer components.

The polyimide film which is the precursor of the carbon film of thepresent invention may contain at least one other high molecular weightcomponent, and may be used in the form of a composite film. Anyconventional high molecular weight component which is soluble in thepolyamic acid solution can be used to provide the composite film.Examples of the high molecular weight component include polyamic acids(a polyamic acid mainly comprising of components (A) and (B) or (A), (B)and (C) but being different combination thereof or proportion thereoffrom that of the polyamic acid which is the precursor of the basicpolyimide may also be used), polyimides, polyamideimides,polyetherimides, polybenzimidazole, polybenzoxazole, polybenzthiazole,aromatic polyamides and polyacrylonitrile. It is preferred that thesehigh molecular weight components have properties which enable them to becarbonized by a high temperature treatment for carbonization andgraphitized.

The amount of the high molecular weight component to be blended varies,depending on the type of the high molecular weight components to beblended, but it is preferred to add an amount which does not have anadverse effect on the strength and modulus of elasticity of thepolyimide film. Preferably, the high molecular weight component is usedin an amount of not more than 20% by weight based on the entire weightof the polyimide film. These high molecular weight components includevarious modified polymers, copolymers, precursors and oligomers. Whenthe high molecular weight components are precursors or oligomers, theymay be formed into a high molecular weight component by, for example, apolymerization after a composite is formed.

The high molecular weight component can be introduced into the polyimidefilm by incorporating an organic solution of the component into thepolyamic acid solution.

The tensile strength and the tensile modulus of elasticity of thepolyimide film which is the precursor of the carbon film can beincreased by stretching, (a) a polyamic acid film composed of theabove-described monomers, (b) a polyamic acid film wherein the polyamicacid is partially imidized (usually not more than 10% of amido groups),(c) a polyimide film which does not contain the other polymer or (d) apolyimide composite film containing the other polymer. Stretching can beconducted as described above.

A polyamic acid film obtained from components (A), (B) and (C) throughhigh-molecular weight gelation of the polyamic acid readily providespolyamic acid film having a very high degree of orientation bystretching, especially by uniaxial stretching. The polyimide filmobtained by the dehydration-cyclization reaction of such an orientatedpolyamic acid film has a high-strength and high-modulus of elasticityand can be preferably used as a polyimide film in the invention.

A polyimide film prepared from a polyamic acid obtained by thepolymerization reaction of monomers comprising pyromellitic dianhydride(component (A)), and p-phenylenediamine (component (B)) and comprisingat least 80% (based on the entire repeating units of the polymer) of arepeating unit of poly(p-phenylene-pyromellitic imide), represented bythe following general formula (V), is preferred. ##STR7## A polyimidehaving the repeating unit represented by formula (V) is highlycrystalline and has a rigid polymer molecular chain.

A film of a polyimide wherein at least 80% of the repeating units arethose represented by formula (V) is an especially superior polyimidefilm for preparing a carbon film having a well-grown graphite layer, ahigh tensile strength, a high modulus of elasticity, a high electricconductivity, and a high density.

By carbonization, the polyimide film undergoes a cyclization reactioninvolving the cleavage and recombination of the molecular chain duringthe course of the carbonization. In this case, --CO--and --N═ bondscontained in the molecular structure which forms the polyimide are takenout as CO₂ and HCN gases during the course of the carbonization, and theremainder proceeds to graphitization.

The polyimide film mainly comprising a poly(p-phenylene-pyromelliticimide) structure has rigid molecular chains and has a nearly linearmolecular structure. Thus, the polymer is highly crystalline and themolecular chains of the polymer are closely packed. Further, thepolyimide film contains relatively fewer molecular bonds such as etheror ester bonds (e.g., --S₂ --, --C₂ --, --O--, --S--) which interferewith the carbonization.

In the polyimide having such a structure, undesired elements arescarcely incorporated therein during the course of the carbonization.The reaction mechanism thereof into a graphitization structure by there-combination of the molecular chain can be simplified, the degree ofcarbonization is high, even at a relatively low carbonizationtemperature, and a structure closely allied to graphite can be easilyformed. Accordingly, in the process using the polyimide film, the degreeof carbonization is high, and the density is high, so a carbon filmhaving excellent mechanical properties, such as a high strength and ahigh modulus can be obtained.

To prepare a carbon film having a high tensile strength, a high tensilemodulus of elasticity, and a high electric conductivity, it is preferredthat the molecular structure of the polyimide be a 100%poly(p-phenylene-pyromellitic imide) structure. However, since thepolyimide having such a structure is too highly crystalline, theresulting film is brittle, its tensile strength is not sufficient, andhandleability as a polyimide film is poor, and as a result the filmeasily be cracked or broken during handling, and it is difficult toobtain a polyimide film having a large area in an industrial level.

A polymer blend or a copolymer is generally used as a means forimproving the brittleness of the highly crystalline polyimide film. Thecopolymerization of the poly(p-phenylene-pyromellitic imide) structureis proposed by JP-A-64-254131. The specification thereof discloses thatit is preferred the amount of the poly(p-phenylene-pyromellitic imide)structure should be not more than 80% by weight to improve thebrittleness of the polyimide film and to obtain a film having excellentmechanical properties. Accordingly, the moldability of the film isconsiderably reduced by the introduction of the rigid chain structure.

However, as a result of investigation it has been found that a filmcomprising polyimide wherein at least 80% of repeating units are thoseof the rigid poly(p-phenylene-pyromellitic imide) structure provides ahigh-performance carbon film. It is very difficult to prepare a highlyrigid polyimide film. However, it is possible to prepare a large-areapolyimide film without brittle fracture, even when the film comprises apolyimide wherein at least 80% of the repeating units are those ofpoly(p-phenylene-pyromellitic imide), by improving the moldability ofthe film by introducing a polyfunctional monomer into the polyimide asdescribed in JP-A-3-146524 (corresponding to U.S. patent applicationSer. No. 07/586103 filed Sep. 21, 1990 and EP 418889A) or improving themolding stage of the film, for example, by gradually imidizing thepolyamic acid film taking a longer time of period.

Examples of monomers which can be preferably introduced into thepolyimide as other monomer component than monomer components of thepoly(p-phenylene-pyromellitic imide) include tetracarboxylic aciddianhydrides such as biphenyltetracarboxylic acid dianhydride andbenzophenonetetracarboxylic acid dianhydride and aromatic diamines suchas m-phenylenediamine, benzidine, o-toluidine,3,3'-dichloro-4,4'-diaminobiphenyl, 4,4'-diaminodiphenylmethane and4,4'-diaminodiphenyl ether. These monomers usually is introduced intothe polyimide in an amount of 1 to 20 mol % based on the total moles ofcomponents. These monomers are introduced to the polyimide by addingthem into the reaction mixture for preparation of the polyamic acid.

Particularly, it is preferred that component (C) such as3,3',4,4'-tetraaminobiphenyl tetrahydrochloride, is also used in thepolyamic acid to obtain poly(p-phenylene-pyromellitic acid).

When the strength of the polyimide film is not sufficient, it is alsopreferred that the imidation is conducted gradually at 100° to 150° C.for 4 to 5 hours while providing 10 to 30% shrinkage to the film.

The carbon film of the present invention is obtained by carbonizing thepolyimide film by heating it in an inert gas such as argon, helium,nitrogen and hydrogen, or in a vacuum (preferably 1 mmHg or less). Theheating may be conducted at 800° C. or higher. The heating is usuallyconducted at 1,000° C. or higher, and preferably at 1,200° C. or higher.In order to avoid curling or shrinking of the film during heating at1000° C. or higher, it is preferred that the film be preheat-treated.The preheating is usually conducted at 500° to 800° C., preferably at600° to 700° C. for 60 minutes. The preheat-treatment is preferablyconducted until the weight reduction of the film reaches at least 40 to50% by weight.

When the highly oriented, high-strength, high-modulus polyimide filmhaving a highly ordered molecular chain structure is used, thecarbonization proceeds in a state in which the polymer molecular chainis arranged in one direction, and hence there can be obtained ahigh-performance carbon film in which the graphite layer is highlyordered, particularly in the molecular chain orientation direction. Inthis case, even when the carbonization treatment is carried out at arelatively low temperature of less than 2000° C., a carbon film can beformed which has a well-grown graphite layer, a high strength and a highmodulus of elasticity, and exhibits excellent electrical conductivity.

In a carbon film obtained by carbonizing a polyimide film which does notsatisfy the mechanical strength requirements of the present invention,the polymer molecular chain is relatively randomly oriented.

Namely, in the carbon film of the present invention, a film is used inwhich the polymer molecular chain of the precursor is highly oriented,mainly in one direction. Hence, crosslinking between neighboringmolecules at a treating temperature (usually of not lower than 1000°C.), that is, graphitization by the cyclization reaction is very easilyconduced, and a high-performance carbon film may be produced in whichthe graphite layer is highly ordered in the orientation direction of themolecular chain. Such a structure is different in the growth directionand arrangement of the graphite layer from the carbon film prepared fromthe precursor in which the arrangement of the molecular chain is random.In the present invention, by raising the carbonizing temperature, carbonfilm having a well-grown graphite layer which provides high-performancecan be obtained. However, even at a relatively low carbonizationtemperature, such as lower than 2000° C., carbon film havinghigh-performance as disclosed hereinabove can be obtained.

It is preferred that the polyimide film which is to be carbonized beplaced between highly heat-resistant sheets, such as graphite sheets orquartz glass sheets, and subjected to the preheat-treatment or theheat-treatment for carbonization. Thus, a change in shape, such as thetwisting or warpage of the film due to shrinkage of the film caused byheat decomposition, can be prevented, and a carbon film having arelatively smooth surface can be obtained. During these heat-treatment,usually no tension is applied to the film.

It is necessary that the film be heated at such a heating rate that thepolyimide film is not deteriorated during treatment, for example, voidsor wrinkle is not caused. It is preferred that the temperature be raisedto 1000° C. at a heating rate of not greater than 10° C./min, preferablyfrom 1 to 5° C./min, and in the treatment at 1000° C. or higher, thetemperature be raised at a heating rate of not greater than 20° C./min,preferably from 1° to 5° C/min.

A carbon film having a higher tensile strength, a higher tensile modulusof elasticity, and a higher electrical conductivity can be obtained bycarbonizing the polyimide film at a higher temperature.

When the heat-treating temperature is higher than 3500° C., a markedchemical change such as sublimation of the carbon component in the filmtakes place, and the film becomes brittle. Hence, a temperature ofhigher than 3500° C. is not preferred. The preferred heat-treatingtemperature is 3000° C. or lower.

In the carbonization treatment, a carbonization catalyst may beemployed. Not more than 20% by weight, preferably 1 to 5% by weightbased on the total weight of polyimide film. A compound of a metalelement, such as, a Group IVa, Va or VIII element of the Periodic Table,for example, Ge, Sn, As, Sb, Fe, Co, Ni, Ru, Rh, Pb, Pt or Pd may beused as a catalyst for carbonization. Examples of the metal compoundinclude a metal chloride such as ferrous chloride, nickel chloride orcobalt chloride.

It is preferred that the catalyst for carbonization, preferably in amolecular state, be dispersed in the polyimide film by incorporating thecarbonization catalyst, for example, into a polyamic acid solution.Then, the carbonization treatment is carried out. By using thecarbonization catalyst the degree of graphitization increases and ahomogeneous carbon film can be easily obtained.

The graphite layer of the carbon film of the present inventionpreferably is doped with a specific material, that is, with a dopantwhich forms a intercalated carbon film, to thereby provide a carbon filmhaving a higher electrical conductivity. The carbon film of the presentinvention can be doped by any conventional method. Examples of dopingmethods include a method wherein a dopant capable of forming anintercalated carbon film in the carbon film is directly brought intocontact with the film in a vapor phase or in a liquid phase, and amethod wherein a dopant capable of forming an intercalated carbon filmis introduced into the film by means of an electrochemical method.

Examples of the dopant which is effective in enhancing the electricalconductivity of the carbon film include alkali metals, halogen compoundsand Lewis acids. Specific examples of the dopant include KBr KI, FeCl₃,CuCl₅, AsF₅, SbF₅, HNO₃ and H₂ SO₄.

The amount of the dopant to be incorporated in the carbon film ispreferably 0.1 to 150% based on the total weight of the carbon filmexcluding the dopant, though there is no particular limitation withregard to the amount of the dopant.

The carbon film of the present invention has metallic luster, a tensilestrength of not less than 15 Kgf/mm² which is preferably not less than20 Kgf/mm², more preferably not less than 25 Kgf/mm², a tensile modulusof elasticity of not less than 5,000 Kgf/mm² which is preferably notless than 8,000 Kgf/mm², more preferably not less than 10,000 Kgf/mm²,and an electrical conductivity of not less than 200 S/cm which ispreferably not less than 280 S/cm, more preferably not less than 300S/cm. The carbon film has a high density which is usually at least 1.70g/cm³.

According to the present invention the tensile strength can be raised upto about 100 Kgf/mm², the tensile modulus of elasticity can be raised upto about 20,000 Kgf/mm² and the electrical conductivity can be raised upto about 2,000 S/cm.

The amount of carbon in the carbon film is preferably at least about 85wt% and the amount of each of hydrogen, oxygen and nitrogen is 10 wt %or less. The amount of carbon is more preferably at least 90 wt%, andmost preferably at least 95 wt %. This element analysis is based on thesubstance derived from the polymer(s) in the carbon film.

The carbon film of the present invention can be used in the fields ofelectrode materials, heating elements, structural materials, heatinsulating materials, heat-resistant sealing materials, parts for X-rayapparatuses, etc. Accordingly, the carbon film has many industrial usesand is a useful material.

EXAMPLE 1

In a 300 ml four-necked separable flask, there were placed 0.0184 mol(1.989 g) of purified p-phenylenediamine (abbreviated as PPD) and 0.0008mol (0.288 g) of 3,3',4,4'-tetraaminobiphenyl tetrahydrochloridedihydrate (TABT). To the flask was then added 50 g of distilledN-methyl-2-pyrrolidone (abbreviated to NMP). The mixture was stirred todissolve them.

The temperature of an external water bath was controlled to 5° C., andin a nitrogen atmosphere, while stirring the solution obtained above,0.02 mol (4.366 g) of purified anhydrous pyromellitic dianhydride (PMDA)in the form of a solid was gradually added thereto with care so that thetemperature of the solution was not raised.

After all of PMDA was added, stirring was continued for 20 minutes. Theuniform solution of a polyamic acid obtained was cast on a glass sheet.The coating amount of the solution was controlled by a spacer to 460g/m². On standing for 60 minutes, the cast polymer solution was gelledand a gel film of the polyamic acid was obtained.

The resulting gel film of the polyamic acid was free-standing.

The gel film of the polyamic acid was dried at 30° C. under vacuum toadjust the content of the solvent in the film to 20% by weight based onthe total weight of the film.

The polyamic acid film was fixed to a steel frame and stepwiseheat-treated at 100° C. for one hour, at 200° C. for one hour, at 300°C. for one hour and at 400° C. for one hour to obtain a polyimide film.The heating rate in the heat-treatment was 5° C./min. The film was foundto have a tensile strength of 18 Kgf/mm² and a tensile modulus ofelasticity of 800 Kgf/mm². Therefore, the film has a high strength and ahigh modulus.

The polyimide film was put between two graphite sheets and treated in anitrogen gas atmosphere in a carbonization furnace at such a heatingrate that the temperature was raised to 600° C. at a heating rate of0.5° C./min, further raised to 1000° C. at a heating rate of 1.5°C./min, and then the film was heat-treated at 1000° C. for 90 minutes.

The film was gradually cooled to room temperature and removed from thecarbonization furnace. The resulting carbon film had a thickness of 40μm and metallic luster and was found to have a tensile strength of 15Kgf/mm², a tensile modulus of elasticity of 5,000 Kgf/mm², an electricconductivity of 250 S/cm, and density of 1.78 g/cm³. Accordingly, it isclear that the film exhibits excellent mechanical characteristics, aswell as electrical conductivity.

The carbon film has a carbon content of 96%, a nitrogen content of 3.0%,and a hydrogen content of 0.5% by weight.

EXAMPLE 2

A polyamic acid film was prepared in the same manner as Example 1. Thethus prepared film was stretched uniaxially to 2 times original lengthand then the film was imidized in the same manner as Example 1. Theresulting film was found to have a tensile strength of 45 Kgf/mm² and atensile modulus of 5000 Kgf/mm². The film was carbonized in the samemanner as in Example 1 to obtain a carbon film. The carbon film wasfound to have a tensile strength of 50 Kgf/mm², a tensile modulus of10000 Kgf/mm², an electric conductivity of 350 S/cm and density of 1.72g/cm³. The carbon content was 95%, a nitrogen content was 3.5%, and ahydrogen content was 0.5% by weight.

These test results show that by stretching the polyamic acid film, acarbon film having higher performance can be obtained.

Wide-angle X-ray diffraction photographs were taken by allowing X-raysto be incident upon the plane of the carbon film from the directionsperpendicular to and parallel with the plane of the carbon film. Adiffraction pattern from the (002) lattice appeared by strong reflectionin the equatorial direction, and it became clear that the graphite layerwas grown in the orientation direction of the film.

COMPARATIVE EXAMPLE 1

Commercially available polyimide film (trade name: Kapton H,manufactured by Toray Du Pont) having a tensile strength of 18 Kgf/mm²and a tensile modulus of elasticity of 300 Kgf/mm² was put between twographite sheets and treated in a nitrogen gas atmosphere in acarbonization furnace at such a heating rate that the temperature wasraised to 800° C. at a heating rate of 1.0° C./min and further raised to1000° C. at a heating rate of 1.5° C./min and then the film was heatedat 1000° C. for 90 minutes. The film was gradually cooled to roomtemperature to obtain a carbon film.

The carbon film taken out from the carbonization furnace had a thicknessof 40 μm and metallic luster and was found to have a tensile strength of20 Kgf/mm², a tensile modulus of elasticity of 3000 Kgf/mm², an electricconductivity of 150 S/cm, and a density of 1.68 g/cm³.

Wide-angle X-ray diffraction photographs of the carbon film were takenby allowing X-rays to be incident upon the plane of the carbon film fromthe directions perpendicular to and parallel with the plane of thecarbon film. A diffraction pattern from the (002) lattice appeared asDebye ring, and it was found that the graphite layer was randomly grownin the film.

In the above-described commercially available polyimide film, themolecular chain which constitutes the polyimide is slightly crystalline,and the film is not highly oriented. The polyimide film has a lowstrength and a low modulus, and hence the resulting carbon film does nothave excellent mechanical and electrical properties.

EXAMPLE 3

Pyromellitic dianhydride, p-phenylenediamine and3,3'-dichloro-4,4'-diaminobiphenyl were subjected to an additionpolymerization reaction in the same manner as Example 1 to prepare apolyamic acid solution. The monomers were used in such a proportion that0.0108 mol of p-phenylenediamine and 0.0092 mol of3,3'-dichloro-4,4'-diaminobiphenyl were used, each amount being based on0.02 mol of pyromellitic dianhydride.

A polyamic acid film was prepared from said polymer solution by means ofa solution casting method in the same manner as Example 1 except thatthe coating amount was 250 g/m². The polyamic acid film was uniaxiallystretched 1.5 times, fixed to a steel frame and stepwise heat-treated at100° C. for one hour, at 200° C. for one hour, at 300° C. for one hourand at 400° C. for one hour to obtain a polyimide film. The film wasfound to have a tensile strength of 30 Kgf/mm² and a tensile modulus ofelasticity of 2500 Kgf/mm², and hence the film had a high strength and ahigh modulus of elasticity.

The polyimide film was put between two graphite sheets and treated in anitrogen gas atmosphere in a carbonization furnace at such a heatingrate that the temperature was raised to 700° C. at a heating rate of 5°C./min, and the film then was heat-treated at 700° C. for one hour.Subsequently, the temperature was raised to 1500° C. at a heating rateof 2° C./min, and the film was heat-treated at 1500° C. for 60 minutes.The film was gradually cooled to room temperature to obtain a carbonfilm.

The carbon film taken out from the carbonization furnace had a thicknessof 20 μm and metallic luster and was found to have a tensile strength of45 Kgf/mm², a tensile modulus of elasticity of 7500 Kgf/mm², an electricconductivity of 280 S/cm, and density of 1.70 g/cm³. The film had acarbon content of 96%, a nitrogen content of 2.0%, and a hydrogencontent of 0.5% by weight. It is clear that the film has excellentmechanical characteristics as well as electrical conductivity.

The wide-angle X-ray diffraction photographs of the carbon film weretaken by allowing X-rays to be incident upon the plane of the carbonfilm from the directions perpendicular to and parallel with the plane ofthe carbon film. A diffraction peak from the (002) lattice appeared bystrong reflection in the equatorial direction, and it was found that thegraphite layer was grown in the stretching direction of the film.

EXAMPLE 4

3,3'4,4'-Biphenyltetracarboxylic acid dianhydride and p-phenylenediaminein an equimolar ratio were subjected to a polyaddition reaction in thesame manner as Example 1 except that the tetramine was not used andN,N-dimethylacetamide (DMA) was used instead of NMP, to prepare a 15 wt%polyamic acid solution (1).

Separately, pyromellitic dianhydride and p-phenylenediamine in anequimolar ratio were subjected to a polyaddition reaction in the samemanner as above to prepare a 15 wt% polyamic acid solution (2).

In order to prepare a blend film from the polyamic acid solution (1) andthe polyamic acid solution (2), the solution (1) and the solution (2) inan equal ratio (percent by weight) were mixed to prepare a mixedpolyamic acid solution composed of the solution (1) and the solution(2).

A blend film of the polyamic acids was prepared from the mixed polymersolution in the same manner as Example 1. The polyamic acid filmcontaining 5 wt% of the solvent was uniaxially stretched 1.5 times atroom temperature, fixed to a steel frame and stepwise treated at 100° C.for one hour, at 200° C. for one hour, at 300° C. for one hour and at400° C. for one hour to obtain a polyimide film. The polyimide film wasfound to have a tensile strength of 45 Kgf/mm² and, a tensile modulus ofelasticity of 3500 Kgf/mm², and hence the polyimide film had a highstrength and a high modulus.

The polyimide film was put between two graphite sheets and treated in anitrogen gas atmosphere in a carbonization furnace at such a heatingrate that the temperature was raised to 800° C. at a heating rate of 2°C./min and then raised to 2000° C. at a heating rate of 5° C./min, andthe film was heated at 2000° C. for 60 minutes. The film was graduallycooled to room temperature to obtain a carbon film.

The carbon film removed from the carbonization furnace had a thicknessof 20 μm and metallic luster and was found to have a tensile strength of45 Kgf/mm², a tensile modulus of elasticity of 14000 Kgf/mm², anelectric conductivity of 450 S/cm, and density of 1.75 g/cm³. The filmhad a carbon content of 98%, a nitrogen content of 0.5%, and a hydrogencontent of 0.2% by weight. It is clear that the film exhibited excellentmechanical characteristics as well as electrical conductivity.

The film was immersed in a nitromethane solution containing 10% byweight of FeCl₃ at room temperature for 24 hours to carry out doping.After doping, the film had an electric conductivity of as high as 8500S/cm.

EXAMPLE 5

Pyromellitic dianhydride and p-phenylenediamine in an equimolar ratiowere subjected to a polyaddition reaction in the same manner as Example1 except that the tetramine was not used, to prepare a 15 wt % polyamicacid solution (3).

Separately, pyromellitic dianhydride, p-phenylenediamine and3,3',4,4'-tetraaminobiphenyl tetrachloride were used as monomers andpolymerized in the same manner as Example 1 to prepare a 15 wt% polyamicacid solution (4). The monomers of the solution (4) were used in such aproportion that 92 mol % of p-phenylenediamine and 4 mol % of3,3',4,4'-tetraaminobiphenyl tetrachloride were used, each amount beingbased on 100 mol of pyromellitic dianhydride.

The solutions (3) and (4) in an equal ratio (percent by weight) weremixed, and a blend film of the polyamic acids was prepared from themixed polyamic acid solution through a high-molecular polymer gel in thesame manner as Example 1.

The film was uniaxially stretched 2.5 times at room temperature, fixedto a steel frame and stepwise heat-treated at 100° C. for one hour, at200° C. for one hour, at 300° C. for one hour and at 400° C. for onehour to obtain a polyimide film. The film was found to have a tensilestrength of 50 Kgf/mm² and a tensile modulus of elasticity of 7500Kgf/mm², and hence had great mechanical strength and a high modulus.

The polyimide film was placed between two graphite sheets and treated ina nitrogen gas atmosphere in a carbonization furnace at such a heatingrate that the temperature was raised to 800° C. at a heating rate of 1°C./min and then raised to 1500° C. at a heating rate of 2° C./min, andthen heat-treated at 1500° C. for 60 minutes. The film was graduallycooled to room temperature to obtain a carbon film.

The carbon film removed from the carbonization furnace had a thicknessof 40 μm and metallic luster and was found to have a tensile strength of60 Kgf/mm², a tensile modulus of elasticity of 18000 Kgf/mm², anelectric conductivity of 800 S/cm, and a density of 1.73 g/cm³. Thecarbon film had a carbon content of 96%, a nitrogen content of 2.5%, anda hydrogen content of 0.3% by weight.

It is clear that the film had excellent mechanical characteristics aswell as electrical conductivity.

EXAMPLE 6

In a 300 ml separable flask were weighed 1.730 g (0.016 mol) of purifiedp-phenylenediamine (PPD) and 0.801 g (0.004 mol) of 4,4'-diaminodiphenylether (4,4'-DPE), and 50.6 g of distilled N,N-dimethylacetamide (DMAc)was added thereto. The mixture was stirred to dissolve them.

The temperature of an external water bath of the flask was controlled to5° C. and while stirring the resulting solution in a nitrogen gasatmosphere, 4.366 g (0.02 mol) of purified anhydrous pyromelliticdianhydride as a solid was added thereto with care so as not to allowthe temperature of the solution to be raised. After completion of theaddition, stirring was continued to carry out a polyaddition reactionfor 1 hour, whereby a uniform polyamic acid solution was prepared.

The polyamic acid solution was stirred at room temperature (about 20°C.) for one hour to complete the reaction. The polymer solution was caston a glass plate. The coating amount of the solution was controlled bymeans of a spacer so as to give a thickness of about 0.5 mm.

The polyamic acid solution cast on the glass plate was dried at 30° C.in a vacuum to prepare a polyamic acid film. After the amount of thesolvent in the film was adjusted to 5% by weight based on the totalweight of the film, the film was peeled off from the glass plate anddried at 80° C. in a dryer for 60 minutes. The film was then fixed to asteel frame and dried at 120° C. for 60 minutes and then at 150° C. for60 minutes. The steel frame was removed, and the film was dried at 200°C. for 60 minutes, again fixed to a steel frame and heat-treated at 300°C. for 60 minutes and at 400° C. for 60 minutes to obtain a uniformpolyimide film.

It was found that the repeating unit (formula (V) ofpoly(p-phenylene-pyromellitic imide) accounted for 80% of the totalrepeating units of the polymer, and the film had a thickness of about 50μm.

The polyimide film was placed between two graphite sheets and treated ina nitrogen gas atmosphere in a carbonization furnace at such a heatingrate that the temperature was raised to 600° C. at a heating rate of0.5° C./min and further raised to 1200° C. at a heating rate of 1.5°C./min, and the film then was heat-treated at 1200° C. for 90 minutes.

The film was gradually cooled to room temperature and taken out from thecarbonization furnace. The resulting carbon film had a thickness of 40μm and metallic luster. The yield was 50%.

The density of the carbon film was measured by means of a sink-floatmethod using a mixed solvent of bromoform and ethanol. The carbon filmwas found to have a density of 1.77 g/cm³. Further, the tensilecharacteristics of the carbon film were evaluated by a test piece of 5mm in width with a distance between chucks being 30 mm. The carbon filmwas found to have a tensile strength of 30 Kgf/mm² and a tensile modulusof 5,500 Kgf/mm².

The carbon content of the carbon film was determined by elementalanalysis. The carbon content was 95%, the nitrogen content was 3.5%, andthe hydrogen contents was 0.3% by weight.

COMPARATIVE EXAMPLE 2

In a 300 ml separable flask were weighed 1.081 g (0.01 mol) of purifiedPPD and 2.002 g (0.01 mol) of 4,4'-DPE, and 54.6 g of distilled DMAc wasadded thereto. The mixture was stirred to dissolve them.

In the same manner as in Example 6, the temperature of an external waterbath of the flask was controlled to 5° C. in a nitrogen gas atmosphere.

While stirring the resulting solution, 4.366 g (0.02 mol) of purifiedanhydrous pyromellitic dianhydride as a solid was gradually addedthereto to carry out a polyaddition reaction for one hour.

The polyamic acid solution was stirred at room temperature for one hourto complete the reaction. The polymer solution was cast on a glassplate. The coating amount of the solution was controlled by means of aspacer so as to give a thickness of about 0.5 mm.

In the same manner as in Example 6, a film was prepared and dried toobtain a polyimide film. The repeating unit of formula (V) accounted for50% of the entire repeating units of the polymer.

The film was carbonized under the same conditions as those of Example 6to obtain a carbon film. The carbon film was found to have a density of1.65 g/cm³, a tensile strength of 30 Kgf/mm², a tensile modulus ofelasticity of 4000 Kgf/mm², an electric conductivity of 180 S/cm, acarbon content of 92%, a nitrogen content of 3.5%, and a hydrogencontent of 1.5% by weight.

COMPARATIVE EXAMPLE 3

In a 300 ml separable flask were weight 2.162 g (0.02 mol) of purifiedPPD, and 65.1 g of distilled DMAc was added thereto. The mixture wasstirred to dissolve it.

In the same manner as in Example 6, the temperature of an external waterbath of the flask was controlled to 5° C. in a nitrogen gas atmosphere,and while stirring the resulting solution, 5.880 g (0.02 mol) ofpurified biphenyltetracarboxylic acid dianhydride (BPDA) as a solid wasgradually added thereto to carry out a polyaddition reaction for onehour.

The polyamic acid solution was stirred at room temperature for one hourto complete the reaction. The polymer solution was cast on a glassplate. The coating amount of the solution was controlled by means of aspacer so as to give a thickness of about 0.5 mm.

In the same manner as in Example 6 a polyimide film was obtained fromthe thus prepared film. The repeating unit of formula (V) accounted for0 mol % of the whole of the polymer. The film had a tensile strength of20 Kgf/mm² and a tensile modulus of elasticity of 500 Kgf/mm².

The film was carbonized under the same carbonization conditions as thoseof Example 6 to obtain a carbon film. The carbon film was found to havea density of 1.70 g/cm³, a tensile strength of 25 Kgf/mm², a tensilemodulus of elasticity of 4500 Kgf/mm² a carbon content of 92%, anitrogen content of 4.0%, and a hydrogen content of 1.5% by weight.

EXAMPLE 7

In a 300 ml four-necked separable flask were weighed 1,730 g (0.016 mol)of purified PPD, and 38.3 g of distilled DMAc was added thereto. Themixture was stirred to dissolve them. The temperature of an externalwater bath was controlled to 5° C. in a nitrogen gas atmosphere. Whilestirring the resulting solution, 3.493 g (0.016 mol) of purifiedanhydrous pyromellitic dianhydride as a solid was added thereto withcare so as not to allow the temperature of the solution to be raised, tocarry out a polyaddition reaction for one hour, whereby a uniformpolyamic acid solution was prepared.

Separately, 0.432 g (0.004 mol) of purified PPD was weighed in anotherflask, and 11.8 g of distilled DMAc was added thereto. Subsequently, inthe same manner as that described above, 1.176 g (0.004 mol) of3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) was addedthereto to carry out a polyaddition reaction for one hour, whereby apolyamic acid solution was prepared.

The above two polyamic acids comprising PMDA/PPD and BPDA/PPD were mixedwith stirring at room temperature in a nitrogen gas atmosphere to obtaina uniform mixed polyamic acid solution.

In the same manner as in Example 6, a film was prepared, dried andheat-treated to obtain a polyimide film. The polyimide contained therepeating units of formula (V) in an amount of 80% based on the totalrepeating units of the polymer, and it had a tensile strength of 25Kgf/mm² and a tensile modulus of elasticity of 800 Kgf/mm².

In the same manner as in Example 6, the film was carbonized to obtain acarbon film. The carbon film was found to have a density of 1.78 g/cm³,a tensile strength of 25 Kgf/mm², a tensile modulus of elasticity of5500 Kgf/mm² and a carbon content of 95% by weight.

EXAMPLE 8

In a 300 ml four-necked separable flask were weighed 1.989 g of purifiedPPD and 0.288 g of 3,3',4,4'-tetraaminobiphenyl tetrahydrochloride, and48.7 g of distilled NMP was added thereto. The mixture was stirred todissolve them.

The temperature of an external water bath of the flask was controlled to0° C. in a nitrogen gas atmosphere. While stirring the resultingsolution, 4.366 g of purified anhydrous pyromellitic dianhydride as asolid was gradually added thereto with care so as not to allow thetemperature of the solution to be raised.

While the temperature of the polyamic acid solution was controlled to 0°C., the solution was stirred for 0.5 hours to obtain a uniform solution.The solution was cast on a glass plate. The coating amount of thesolution was controlled by means of a spacer so as to give a thicknessof about 0.5 mm.

The polyamic acid solution cast on the glass plate was caused to gelafter a while, whereby a uniform gel film was obtained.

The polyamic acid gel film was dried in a vacuum at 45° C. for 3 hoursand at 80° C. for 60 minutes to obtain a polyamic acid film containingNMP in an amount of 2.0% by weight which was then peeled off from theglass plate. Further, the film was fixed to a steel frame and dried in adryer at 100° C. for 60 minutes and at 150° C. for 60 minutes. The filmwas then heat-treated at 200° C. for 60 minutes, at 300° C. for 60minutes and at 400° C. for 60 minutes to obtain a uniform polyimidefilm. The polyimide film was put between two graphite plates and treatedin a nitrogen gas atmosphere in a carbonization furnace at such aheating rate that the temperature was raised to 800° C. at a heatingrate of 0.5° C./min and further raised to 1000° C. at a heating rate of1.5° C./min, and the film was heated at 1000° C. for 90 minutes.

The film was gradually cooled to room temperature and taken out from thecarbonization furnace. The resulting film had a thickness of about 40 μmand metallic luster. The yield was 53%.

The carbon film was found to have a density of 1.78 g/cm³, a tensilestrength of 30 Kgf/mm², a tensile modulus of elasticity of 6000 Kgf/mm²,and an electro-conductivity of 280 S/cm.

The carbon content of the carbon film was determined by elementalanalysis. The carbon film was found to have a carbon content of 95%, anitrogen content of 3.0%, and a hydrogen content of 0.6% by weight.

EXAMPLES 9 TO 11

Monomers in amounts indicated in Table 1 were charged into a 300 mlfour-necked separable flask. In the same manner as in Examples 6 to 8,polyimide films were prepared.

Each polyimide film was put between two graphite plates. In the samemanner as in Example 6 to 8, the film was carbonized to the finalheat-treating temperature indicated in Table 1 in the carbonizationfurnace to obtain a carbon film. The temperature was raised to 600° C.at a heating rate of 0.5° C./min and further raised to the finalheat-treating temperature at a heating rate of 1.5° C./min.

The carbon films obtained in these Examples have a high density and highmechanical characteristics as shown in Table 1.

The abbreviation name, chemical formula and molecular weight thereof areshown in Table 2.

                                      TABLE 1                                     __________________________________________________________________________                                                 Comparative                      Type of monomer and                                                                          Example                       Example                          amount (g)     6    7    8    9    10   11   2    3                           __________________________________________________________________________    Acid anhydride                                                                PMDA           4.366                                                                              3.493                                                                              4.366                                                                              4.366                                                                              4.366                                                                              4.366                                                                              4.366                                                                              --                          BPDA           --   1.176                                                                              --   --   --   --   --   5.880                       Aromatic diamine                                                              PPD            1.730                                                                              2.162                                                                              1.989                                                                              1.730                                                                              1.946                                                                              1.838                                                                              1.081                                                                              2.162                       MPD            --   --   --   0.216                                                                              --   --   --   --                          4,4'-DPE       0.801                                                                              --   --   --   --   --   2.002                                                                              --                          OTD            --   --   --   --   --   0.425                                                                              --   --                          DCDH           --   --   --   --   --   0.326                                                                              --   --                          Others                                                                        TAB            --   --   0.288                                                                              0.360                                                                              --   0.360                                                                              --   --                          Polyamide-imide                                                                              --   --   --   --   --   0.750                                                                              --   --                          Type of solvent for                                                                          DMAc DMAc NMP  NMP  NMP  DMF  DMAc DMAc                        polymerization*.sup.1         DMAc                                            Final heat-treating temp. (°C.)                                                       1200 1200 1000 1800 2300 2000 1200 1200                        for forming carbon film                                                       Characteristics of carbon film                                                Density (g/cm.sup.3)                                                                         1.77 1.78 1.78 1.80 1.83 1.80 1.65 1.70                        Tensile strength (Kgf/mm.sup.2)                                                              30   25   30   25   18   25   30   25                          Tensile modulus of                                                                           5500 5500 6000 6000 5800 5700 4000 4500                        elasticity (Kgf/mm.sup.2)                                                     Carbon content (wt %)                                                                        95   95   95   96   97   95   92   92                          Nitrogen content (wt %)                                                                      3.5  3.2  3.0  2.4  1.8  2.0  3.5  4.0                         Hydrogen content (wt %)                                                                      0.3  0.6  0.6  0.4  0.6  0.8  1.5  1.5                         Electric conductivity                                                                        250  270  280  380  550  420  180  190                         __________________________________________________________________________     *.sup.1 Type of solvent for polymerization                                    DMAc: N,Ndimethylacetamide                                                    NMP: Nmethyl-2-pyrrolidone                                                    DMF: N,Ndimethylformamide                                                     Numerals for amounts in Table represent weight (g).                      

                                      TABLE 2                                     __________________________________________________________________________    Classification                                                                         Abbreviation                                                                          Name          Chemical formula          Mol.                 __________________________________________________________________________                                                             wt.                  Tetracarboxylic                                                               acid anhydride                                                                         PMDA    Pyromellitic dianhydride                                                                     ##STR8##                 218                           BPDA    3,3', 4,4'-biphenyltetra- carboxylic acid                                                    ##STR9##                 294                           PPD     p-phenylenediamine                                                                           ##STR10##                108                           MPD     m-phenylenediamine                                                                           ##STR11##                108                           4,4'-DPE                                                                              4,4'-diaminodiphenyl ether                                                                   ##STR12##                200                           OTD     o-toluidine (3,3'-dimethyl- 4,4'-diaminobiphenyl)                                            ##STR13##                212                           DCDH    3,3'-dichloro-4,4'- diaminobiphenyl dihydrochloride                                          ##STR14##                326                           TAB     3,3',4,4'-tetraamino- biphenyl tetrahydro- chloride                                          ##STR15##                360                                   polyamide-imide (Thorone (trade name) manufactured by                         Mitsubishi Kasei Corp.)                                                                      ##STR16##                --                   __________________________________________________________________________

What is claimed is:
 1. A carbon film derived from a polyimide film,which has a tensile strength of at least 15 Kgf/mm², a tensile modulusof elasticity of at least 5000 Kgf/mm² and an electric conductivity ofat least 200 S/cm.
 2. The carbon film as claimed in claim 1, wherein thetensile strength is at most 100 Kgf/mm², the tensile modulus ofelasticity is at most 20,000 Kgf/mm², and the electric conductivity isat most 2,000 S/cm.
 3. The carbon film as claimed in claim 1, whereinsaid polyimide is obtained by imidizing a polyamic acid selected fromthe group consisting of: (1) a polycondensation product of atetracarboxylic acid dianhydride and an aromatic diamine; and, (2) apolycondensation product of a tetracarboxylic acid dianhydride, anaromatic diamine and a polyamino compound having at least three aminogroups.
 4. The carbon film as claimed in claim 1, which has a density ofat least 1.7 g/cm³, a carbon amount of at least 85 wt %, and an amountof each of hydrogen, oxygen and nitrogen of 10 wt % or less.
 5. Thecarbon film as claimed in claim 1, which has a nitrogen amount of atleast 0.5 wt %.
 6. The carbon film as claimed in claim 1, which has anelectric conductivity of at most 800 S/cm.